1. Field of the Invention
This invention generally relates to polymer resistors for use in potentiometers. In particular, the invention is directed to a resistive film that contains nanomaterials.
2. Description of the Related Art
Electrically resistive polymer thick film compositions have numerous applications. Polymer thick film (PTF) resistive compositions are screenable pastes, which are used to form resistive elements in electronic applications. Such compositions contain conductive filler material dispersed in polymeric resins, which remain an integral part of the final composition after processing.
Resistive compositions are used as resistive elements in variable resistors, potentiometers, and position sensor applications. A resistive element is in most cases printed over a conductive element, which acts as the collector element. In position sensing applications, a metallic wiper slides over the resistive element. The wiper can slide back and forth for several million cycles over the collector and resistive elements during the lifetime of the electronic component. For accurate position sensing, the wiper should give continuous electrical output throughout the life of the sensor. The durability of these position-sensing elements depends on the mechanical properties of both the resistor and the conductive film. The polymer thick films tend to wearout after several million cycles of sliding with a metallic contactor moving over the elements at extreme temperature conditions such as in an automotive engine compartment. Polymer resistive and conductive compositions having excellent mechanical properties and wear resistance are required for these applications.
In addition to good mechanical properties, these materials should also have good thermal properties. Polymer thick films show a decrease in storage modulus as temperature is increased. A sharp decrease in mechanical properties is observed near the glass transition temperature. In addition to loss in modulus, these materials also tend to show an increase in coefficient of thermal expansion, which increases significantly above the glass transition temperature. A position sensor is exposed to high temperatures in an engine compartment. At these temperatures, the elements show a high rate of wear due to a decrease in modulus properties. In addition to the surrounding temperature, a still higher temperature is observed at the interface between the metallic wiper and the element surface due to frictional heating. In some cases these temperatures can approach the glass transition temperature (Tg) of the material and can cause loss of the mechanical properties, which adversely affect the signal output.
A prior art resistor composition is as follows:
Prior Art Composition
One way to increase the mechanical properties of the film is to incorporate fillers such as short fibers into the films. The presence of fibers with a relatively large dimension creates electrically heterogeneous surface. This results in non-linear electrical output in contact sensor applications. Even when the fibers are in micron dimension, the surface can be electrically and mechanically heterogeneous. A dither motion at high frequency on a surface region where these fibers are absent can create large wear. Another problem occurs when fibers greater than 10 volume percentage are used. This can significantly wear the metallic contactor. This wear is accelerated if these fibers are protruded from the surface. Therefore, there is a current unmet need for a resistive film with enhanced mechanical and thermal properties with homogeneous surface electrical characteristics
A resistive film for use in a potentiometer. The film is in contact with a movable wiper. The film includes a cured polymer resin and a cured thermosetting resin. Conductive particles of carbon black and graphite are dispersed in the film. The conductive particles cause the resins to be electrically conductive. Nanoparticles are also dispersed in the film. The nanoparticles increase the wear resistance of the resistive film as the wiper moves across the film.